This chapter focuses on five-membered heterocyclic compounds with two or more heteroatoms and discusses their possible structures, nomenclature, properties, and tautomerism. Systematic replacement of CH by N leads to nine additional monocyclic heteroaromatic nitrogen systems, which are known collectively as azoles. Reduction of one or two double bonds in the azole ring gives nonaromatic systems, which have been described as azolines and azolidines. Each of the aromatic azolium cations can be converted into a neutral system by substitution of an anionic O, S, or NR group on to a ring carbon atom. Examples of oxygen and sulfur rings with exocyclic conjugation systems are the 1,2-dithiole-3-thione and the type A mesoionic 1,3-thiazolium-4-olate. Isoxazole, thiazole, and their derivatives are examples of five-membered heterocyclics when ring atoms can be chosen from oxygen, sulfur, and nitrogen. The chapter presents the findings of research studies on the intermolecular forces between five-membered heterocyclic molecules and their influence on melting and boiling points, solubilities, and chromatographic properties, their thermochemistry, conformations of saturated ring systems, and aromaticity. Overall, the aromaticity of azoles is intermediate between that of five-membered heterocycles with one heteroatom and six-membered heterocycles (azines). Furthermore, types of tautomerism in five-membered heterocyclic compounds with two or more heteroatoms, including prototropic tautomerism, ring-chain tautomerism, and valence tautomerism are discussed.
